The present invention relates generally to a static power converter, and a method of exchanging energy and converting power between two electric power circuits, such as a utility grid and a load, and more particularly to a unipolar series resonant converter for use in a high power, multi-kilowatt system for providing direct current (DC) or alternating current (AC) output power from either an AC or a DC power source.
Typically, converters are used to couple an electric load with a power source. For example, converters are used with uninterruptible power supplies, arc furnaces, and induction motor drives. During operation, the converters and their loads generate harmful harmonic currents which can cause voltage spikes on the power utility grid. These spikes can damage the equipment of other customers receiving power from the utility. Computers are especially vulnerable to damage from voltage spikes caused by these harmonic currents.
Filters are often used between the utility grid and the converter, as well as between the converter and the load, but filters are very expensive, both in terms of initial installation and operating costs. For example, a five horsepower induction motor may cost $150, whereas the converter costs $2,000 and the filters $1,000. Thus, engineers have focused on improving converter designs to decrease the initial cost of an induction motor drive installation. A variety of earlier resonant converters are described in various patents and publications, such as the textbook by Mohan, Undeland and Robbins: Power Electronics: Converters, Applications and Design, (John Wiley & Sons, 1989), pages 154-200.
Basically, a traditional resonant converter has input and output switch assemblies coupled together by at least one resonant circuit or "resonant tank." Filters are often coupled to the input and output switch assemblies. The switch assemblies are groups of semiconductor switches, such as (listed in order of increasing cost) diodes, thyristors, gate-assisted turnoff thyristors (GATTs), gate turn off thyristors (GTOs), insulated gate bipolar transistors (IGBTs), and metal oxide semiconductor field effect transistors (MOSFETs).
The resonant circuit of a traditional resonant converter facilitates what has come to be known as "soft switching." In soft switching, the semiconductors are switched at substantially zero current, termed "zero current switching" (ZCS), or at substantially zero voltage, called "zero voltage switching" (ZVS), or with a combination of ZCS and ZVS. As a result, lower switching losses are incurred during soft switching than in traditional "hard" switching schemes, and such low switching losses facilitate faster switching (on the order of 20 kHz, as opposed to one kilohertz with hard switching). Thus, significantly higher switching frequencies are achieved in resonant converters.
The high frequency soft switching capability of resonant converters is often exploited to minimize harmonic distortion of voltage and current waveforms at both the input and output of the converter. High frequency soft switching also dispenses with the need for bulky and expensive low order harmonic filters. Moreover, the size and weight of magnetic and electrostatic components associated with the power electronic energy conversion process are also reduced.
There are many kinds of resonant converters, particularly at the low end of the power spectrum, for example, a few hundred watts or less. However, in high power multi-kilowatt applications, less options are available to the converter designer in the way of devices and sophisticated circuit topologies. Thus, it is more difficult to design a low cost high power converter capable of running at high conversion efficiency.
Two classes of high power resonant converters have demonstrated some success, specifically:
1. Series resonant converters, and PA1 2. Parallel resonant converters. PA1 1. Current flowing through the device is turned off by natural commutation; and PA1 2. The device is subjected to a sufficient back bias voltage for a sufficient duration (turn off time). PA1 U.S. Pat. No. 3,953,779 to Schwarz (1976) PA1 U.S. Pat. No. 4,096,557 to Schwarz (1978) PA1 U.S. Pat. No. 4,495,555 to Eikelboom (1985) PA1 U.S. Pat. No. 4,523,269 to Baker et al. (1985) PA1 U.S. Pat. No. 4,648,017 to Nerone (1987) PA1 U.S. Pat. No. 4,679,129 to Sakakibara et al. (1987) PA1 U.S. Pat. No. 4,695,933 to Nguyen (1987) PA1 U.S. Pat. No. 4,727,469 to Kammiller (1988) PA1 U.S. Pat. No. 4,853,832 to Stuart (1989)
A combination of these classes has been proposed as well. The fundamental difference between these two converter classes is the manner in which power is transferred through the converter to the load. For a parallel resonant converter, the load terminals are in parallel with a resonant capacitor within the resonant tank. For a series resonant converter, the load terminals are in series with the resonant tank capacitor. In either the series or parallel resonant convertors, the load may be coupled either directly to the resonant capacitor, or indirectly through switches and other storage elements.
Conceptionally, the resonant circuit serves as a link between the input and output of the converter. The resonant circuit is controlled to generate a train of pulses which may have constant or varying pulse and cycle widths. The fundamental frequency of these pulses, defined herein as the "link frequency," is chosen to be significantly higher than the frequency of the input and output voltages or currents. The converter receives the input power at an input frequency, and converts the input power into a train of pulses, defined herein as the "link power." This link power is then converted again to obtain the output power at a selected output frequency. Either the input power, the output power, or both may be DC power (that is, power having currents and voltages with zero frequency).
The different topologies of the earlier resonant converters use different kinds of semiconductors. The lowest cost semiconductors are robust controlled rectifiers, also known as thyristors. Thyristors are useful for resonant converters only if two operating conditions are met, specifically, if:
Thyristors are unsuitable in high power resonant converters if the link frequency is so high that the device turn off time leads to an unacceptable duty cycle of the link current or voltage pulses. However, today's thyristors have a frequency ceiling beyond the audible frequency range (about 20 Khz), and are acceptable for most high power applications if the two operating conditions above are met. If either condition is not satisfied, then the more expensive controllable turn off switches such as GTOs, power MOSFETs and IGBTs must be used. As opposed to thyristors which only have a controllable turn on time, the GTOs, MOSFETs and IGBTs all have both controllable turn on and turn off times, activated by simply applying and removing gate driver signals.
In parallel resonant converters, the link pulse train is usually formed by unipolar (or unidirectional) voltage pulses, and usually requires controllable turn off switches which are turned off at substantially zero switch voltage. An example of such a parallel resonant converter is described in the 1989 U.S. Pat. No. 4,864,483 to Divan.
In series resonant converters, the link pulse train is formed by either AC or unipolar (unidirectional) current pulses. Several conventional series resonant converters employing AC link current pulses, are disclosed in the following U.S. Patent Nos.:
The more expensive controllable turn off switches (e.g. GTOs, IGBTs), are not needed because the link pulses are current pulses which cause the thyristors to turn off at substantially zero current, a performance characteristic known as "natural commutation." To accommodate the flow of these AC link current pulses, both the input and output switch assemblies must consist of bi-directional switches, such as two antiparallel coupled thyristors. For example, such a series resonant converter designed for three phase AC input and output with regenerative capability, requires twelve pairs of antiparallel switches. A significant improvement was invented by Klaassens and Lauw, as disclosed in U.S. Pat. No. 5,010,471. By roughly doubling the peak value of the AC link current through the conventional series resonant converter, Klaassens and Lauw replace the full bridge configuration of the input and output switch assemblies with a half bridge configuration. The resulting Klaassens/Lauw converter needs only half the number of switches of a conventional full bridge series resonant converter, whether considering bi-directional or antiparallel pairs of unidirectional switches.
Because series resonant converters employing AC link current pulses use bi-directional switches, or antiparallel pairs of unidirectional switches, saturable inductors must be inserted in series with each switch. The saturable inductors avoid the well known dv/dt turn on disturbances of thyristors, i.e. unscheduled thyristor turn on caused by an excessive rate of change of the anode to cathode voltage. The high number of saturable inductors, as well as the usual parallel capacitive snubbers, both increase the cost, size and volume of the converter. Furthermore, these saturable inductors force the designer to use switches with a higher reverse blocking voltage capability. The designer must also increase the minimum duration of the idle segment of the link current pulse beyond the turn off time as specified by the thyristor manufacturer. Another drawback are the losses incurred during turn on of the switches. These turn on losses occur because the voltages across the switches are not substantially zero when current begins to flow through the switches.
In U.S. Pat. No. 4,942,511 to Lipo and Murai propose a DC link series resonant converter which employs unipolar (unidirectional) link current pulses, rather than AC link current pulses. The Lipo/Murai converter provides a DC current bias to the resonant current pulses. Since the link currents are unipolar, only unidirectional switches are needed. Thus, like the Klaassens/Lauw half bridge series resonant converter, the Lipo/Murai converter only requires half the number of unidirectional switches as needed by conventional series resonant converters.
In the Lipo/Murai converter, even though each pulse cycle of the link current returns to zero naturally, the thyristors of the unidirectional switches are not exposed to a firm back bias voltage. This condition forces the thyristors to be kept at zero current for a duration longer than the manufacturer-specified turn off time. Thus, the Lipo/Murai converter violates the second thyristor operating condition (2) mentioned above. As a result, when compared with a conventional series resonant converter (for equal pulse cycle width and average values over the entire pulse cycle), the Lipo/Murai converter must generate link current pulses with a significantly higher peak value. The other option for the Lipo/Murai converter is to use the more expensive controlled turn off switches, rather than thyristors.
In spite of the superior performance of soft switching series resonant converters over converters employing hard switching circuitry, there still is a need for crucial improvements to series resonant converter technology. For example, one of the most critical barriers to the commercial success of series resonant converters is that the link current pulses must have extremely high peak values. Depending on the type of series resonant converter used, the peak value of the link current pulses may reach three to nine times the peak value of the maximum output current demanded by the load.
This phenomena of high link current pulse peaks stems from the use of sinusoidal current pulses which are generated entirely through resonant oscillation of the resonant circuit. One solution is proposed by Murai, Nakamura, Lipo and Aydemir in the article, "Pulse-split Concept in Series Resonant DC Link Power Conversion for Induction Motor Drives," submitted to the Industrial Application Society Meeting of 1991. Lipo and Murai attempted to improve the converter circuitry by modifying the waveform of the link current pulses as described in their U.S. Pat. No. 4,942,511. The Lipo/Murai converter uses a saturable reactor with a biasing current to limit the peak value of the resonant current pulses. However, the Lipo/Murai converter still causes the thyristors to be operated in violation of the second condition mentioned above, that is, the thyristors are not subjected to a sufficient back bias voltage for a sufficient duration. As a result, the use of thyristors for Lipo/Murai's converter still leads to an excessive ratio of the peak value and average value of the link current pulses. Since the link current is still too high, the Lipo/Murai converter is very expensive because the converter price is directly proportional to the link current value.
U.S. Pat. No. 4,477,868 to Steigerwald discloses another type of series resonant converter which limits the peak value of the link current pulses to moderate values. However, the Steigerwald converter is unfortunately restricted to nonregenerative applications, and only DC input and output power. Moreover, the Steigerwald converter expects the input power to behave as a current source. The Steigerwald converter uses expensive controllable turn off switches (GTOs), rather than thyristors, to convert the DC input current waveform into alternating square waves.
In summary, there are three main types of converters. First came the general linear mode converters, which suffered very high switching losses. Second, resonant converters were developed for high power applications, such as the Schwarz converters. The resonant converters relied on resonant circuits to reduce switching losses, but they still suffered from high peak current losses. The Lipo/Murai resonant unipolar converter falls in this category. Third, quasiresonant converters were developed to take advantage of the best characteristics of both linear and resonant converters, such as the parallel resonant converter developed by Divan. These earlier quasiresonant converters required many expensive controllable turn off switches.
Thus, a need exists for an improved series resonant converter and method of exchanging energy and converting power between single phase, three phase, and/or DC power sources and/or loads, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.